5402: Align nginx image env-var contract

[turegjorup]

Summary

Finalize the nginx image's operator-facing env-var surface — one name per concept, so the compose side never has to guess what to set.

Variable	Before	After	Purpose
NGINX_FPM_SERVICE	host:port combined	os2display (host only)	upstream hostname
NGINX_FPM_PORT	(embedded in NGINX_FPM_SERVICE)	9000	upstream port
NGINX_PORT	8080	8080	listen port (unchanged)
NGINX_MAX_BODY_SIZE	10m	140m	upload cap
NGINX_SET_REAL_IP_FROM	172.17.0.0/16	172.16.0.0/12	trusted-proxy CIDR

Why these defaults

• /16 → /12 for NGINX_SET_REAL_IP_FROM: covers the full RFC1918 docker bridge range (172.16.0.0–172.31.255.255) so multi-network compose stacks don't need to override.
• 140m upload cap: aligns with the planned PHP_POST_MAX_SIZE ceiling for media uploads. Operators raising one and forgetting the other was a frequent footgun.
• Splitting NGINX_FPM_SERVICE and NGINX_FPM_PORT: the previous combined host:port value forced operators to embed the port in the hostname env var, which made it impossible to override one without the other and read awkwardly in compose.

Local validation

Built the production nginx image from this branch and verified at runtime:

$ docker run --rm -e NGINX_MAX_BODY_SIZE=200m … cat /etc/nginx/conf.d/default.conf | grep client_max_body_size
    client_max_body_size 200m;

$ docker run --rm -e NGINX_FPM_SERVICE=os2display -e NGINX_FPM_PORT=9000 … head -4 /etc/nginx/conf.d/default.conf
upstream api_backend {
    server os2display:9000;
    keepalive 16;
}

Out of scope

• The dev compose stack uses its own template under .docker/templates/ and is unaffected. (Unifying dev and prod templates is a separate effort.)

Test plan

• Build image from this branch; docker run -e NGINX_MAX_BODY_SIZE=200m <image> cat /etc/nginx/conf.d/default.conf | grep client_max_body_size shows 200m.
• Without overriding the FPM env vars, the rendered upstream block shows server os2display:9000;.
• No regression in the local dev stack — task compose-up and the dev nginx still serves correctly (separate template).

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